Systems and methods for detecting, sorting, measuring, and imaging particles are useful in a variety of situations. The detection and analysis of the physical properties of biological particles, in particular, is generally referred to as cytometry. In conventional cytometry, a beam of light is projected through a fluid stream containing particles. When struck by the beam of light, the particles give off signals that are picked up by detectors. From these signals, information about the particles can be determined.
The signals generally fall into two categories. The first category is light scattered by the particles. The extent of scattering depends on physical properties of the particles, such as, the size, refractive index, and surface topography of the particles. Conventional cytometry uses multiple lenses placed in strategic locations to collect diffracted, reflected, and refracted light. One lens is typically located near the axis of the incident light to collect forward scattered light. Forward scattered light provides information about on the surface area or size of the particles. Another lens is typically placed at about ninety degrees to the incident light to collect side scattered light. Side scattered light provides information about the internal complexity of the particles. A series of beamsplitters and filters then steers the collected light to appropriate detectors.
The second general category of signals is fluorescence emitted by the particles. In order to measure biological and/or biochemical properties of interest, for example, cells are usually stained with fluorescent dyes that bind to specific portions of certain types of cells. When illuminated, the dyes absorb light energy over a particular range of wavelengths and emit fluorescence at wavelengths generally higher than that of the excitation light. Conventional cytometry collects the fluorescence with the same lens used to collect side scattered light. Fluorescence measurements can be used to identify particles and to provide limited quantitative information about the particles.
The amount of fluorescence emitted is proportional to the amount of dye molecules on the particles. Conventional cytometry lenses, however, detect only a small fraction of the emitted fluorescence. Problems also arise when the size range of the sample particles varies. In such cases, for example, the position of the detection lens may require re-positioning.
Thus, there is a need to overcome these and other problems of the prior art and to provide a particle detector and a method for its use.